theuseofhot-worktoolsteelsatpsa peugeot citroËn … · theuseofhot-worktoolsteelsatpsa peugeot...

THEUSEOFHOT-WORKTOOLSTEELSATPSAPEUGEOT CITROËN

M. PlateauPSA PEUGEOT CITROËN

Belchamp

France

AbstractImproving life time of the forging tools or the pressure die casting tools is

very important for a company which presents a constant increasing productionrate and where an important ratio of parts is produced in its own plants.

Thermomechanical cracking is one of the causes of tool damages. It isa fatigue problem, and improvement of hardness increase fatigue properties.But if the ductility is not good enough thermomechanical cracking will passfrom ductile to a brittle way. So, it is important to have a good ductility atthe origin of steel condition in order to improve hardness.

CNOMO standardisation, using by PSA and RENAULT, fixes difficultlevels on X38CrMoV5.1 and X36CrMoV5.1.

The way of making the heat treatment is important because it influencesbrittleness and ductility.

PSA use nitriding on forging tools and small nitriding on pressure diecasting tools. In pressure die casting tools nitriding has to be controlledvery carefully, but the experiment we have performed shows that excludingnitriding results are not good enough.

In forging PSA has made tests related to welding and materials whichpresent better hardness with a good level of ductility. In pressure die castingPSA are looking for improvement of hardness and ductility, and materialswith better thermal conductivity.

1391

1392 6TH INTERNATIONAL TOOLING CONFERENCE

INTRODUCTION

The only mission of a tool is to produce good parts that signify partscorresponding to a demand clearly asked from the customer. Thus, thecustomer can consider that the problem of tools is under the responsibilityof the supplier. So it is your own problem when the parts are made in yourown plants, all the more when you make the tools used to make these parts.In PSA PEUGEOT CITROËN, we make about 75% of forging parts in ourforging plant. This is the reason of the interest in tools in the company PSA.

More generally, improving life time, of the forging tools orthe pressuredie casting tools, is very important for a company which presents a constantincreasing rate of production. There are two reasons for this:

The first reason, which is the more evident but not the more important,is to decrease the ratio of the cost of the tool / number of parts made bythe tool. The cost of the steel plus the heat treatment is about 30% ofthe global cost of the tool. So it is easy to understand that a better steel,which will give an important improvement of lifetime, will be veryinteresting even if its cost is higher. If we can reduce the cost of toolsin the automotive parts we are selling, we will be more competitive inautomotive business.

The second reason is, that the better your tools lifetime is the bet-ter is your productivity rate. As production time is more andmoreimportant, the time used to change a tool is very expensive.

Working on material issues is the more evident to reach the goal becausethe part design impose the shape of tools and it is not easy to change amanufacturing process. But, some times, there is no choice,and one haveto modify the design and manufacturing process. A material improvementis not the solution to all the problems.

PRODUCTION OF FORGING AND PRESSURE DIECASTING PARTS IN PSA

The cost is the global cost including material, treatment and machiningcosts. Concerning forging: the cost of tools is about 0,17E /Kg produced.Concerning pressure die casting: the cost of tools is about 0,15E /Kg pro-duced. 0,01Eunits represents a real significant economical impact.

The Use of Hot-Work Tool Steels at PSA PEUGEOT CITROËN 1393

Table 1.

Examples of partsVolume of

production/year(Tons)

Cost oftools/year (ME )

Forging gear,crank shaft,triangle,connecting rod

85.000 14.5

Pressure diecasting

crank case,gear box case

61.500 9

It should be noted that the referred costs are valid when 100%of tools aremade in PSA plants.

DAMAGES ON TOOLS

FORGING

There are 2 majors causes of damages:

thermomechanical cracking

wear

We do not experience brittle problems with our tools. This problem ischaracterized by a fast degradation of the tool until the rupture. If thisproblem arise we look for decreasing hardness or tries to choose a materialless brittle.

As PSA work only with mechanical forging, we have less problem withimportant plastic deformation of the tool at high temperatures.

To decrease wear, one have to increase hardness of the tool.To decrease the damage due to thermomechanical cracking will be the

next development.It will be difficult to improve the middle life time for forging tools, because

it depends a lot of the shape of the tools, on the temperature of forging andof the rank of the tools in the process. The lifetime may rangefrom 6.000to 17.000 parts.


PRESSURE DIE CASTING

There is one major cause of damage, which is the thermal cracking.Aluminium sticking (dissolution of steel and sticking of aluminium on

the die) is a problem too, because it creates stress in the tool at the overtureand it can cause damages. Ways of reducing this phenomena areknown butnot so easily to use in a die.

PSA made some tests with different samples immerged in AS7G0.3 (thealuminium alloy used in pressure die casting is AS9U3) at 700℃ with atypical cycle of immersion. Time of immersion / non-immersion is not givenhere because it is not representative of what happened in thedie, presentlythe interest is to compare.

Some results are listed in Table 2, and it was concluded that:

Table 2.

Number of immersionsTesting before important

damages

Alloy with 97% nickel non coated 100Super alloy with a cobalt base non coated 200X38CrMoV5 non coated 200X38CrMoV5 nitrided (reference) 1.600Alloy with molybdenum base non coated 2.900X38CrMoV5 with PVD process (TiN + TiAlN) 4.600X38CrMoV5 with CVD process (TiCN + Al2O3) 4.600

Nickel and liquid aluminium are not compatible

Super alloys are not good to solve the problem

The alloy with molybdenum base is a good solution but it is tooex-pensive

The nitriding reference is a good solution

PVD and CVD coatings used in the tests are the best solutions to theproblem


The CVD and PVD processes present other problems. With the high tem-perature treatment it is impossible to make a CVD-coating onthe PSA dies,which are big dies with complicated shapes, because there will be importantdeformation. Concerning these process there is another problem due to thelow coefficient of deformation admitted by the layers, whichare ceramics.The die moves when it works, and if the basic steel allows deformation, thenthe deformation allowed by ceramics are lower, and crackingwill occur.When the layer is cracked, the liquid aluminium will penetrate the layer andthe end of the tools will arrive fast. The last problem is thatit is very difficultto repair a PVD- or CVD-coated die. One important point should be notedconcerning aluminium sticking, not talked about here, is the lubricant.

CONCERNING THERMAL ANDTHERMOMECHANICAL CRACKING

Thermal and thermomechanical cracking are fatigue phenomena. Fatiguecracking is supposed to be initiated in a volume submitted toplastic defor-mation of limited extent. When the yield strength of a material increasesit will limit the possibility of plastic deformation, whichmay decrease thefatigue cracking phenomena. Thus, increasing the hardnessof a tool steel,increases its resistance to thermal and thermomechanical cracking. This istrue to an ultimate point where the hardness of the steel gives the tool brittleproperties.

So it is really important to have materials which are as ductile as possible(comparatively at a given level of hardness) because it willallow to increasethe hardness without having problems of brittle mode fracture (as grosscracking), and so it is possible to limit the thermal and thermo mechanicalcracking.

STANDARDS

When the aim is to increase the tooling life time, as through the materialperformance one first have to set a good standard and be assurethat thestandard is really well used.


SPECIFICATIONS

The following list is not exhaustive but presents some examples. For eachtool there is a specification of the steel, the hardness and type of nitriding,see Table 3.

Table 3.

Tool Steel Hardness Nitriding

Casting parts on dies X36CrMoV5.1 44–46 HRC slowCrank shaft matrix X38CrMoV5.1 39–42 HRC doublePattern in middle hot forging X38CrMoV5.3 52–54 HRC TeniferMatrix for centring in HATEBUR X38CrMoV5.3 47–49 HRC single

STANDARDISATION

The standard used for the material is called CNOMO. CNOMO groups allthe standardisation on equipment for French automotive construction. Thestandardisation is made through the work of PSA PEUGEOT CITROËN andRENAULT.

PSA do not refer to the NADCA specification, and do not recognize thedesignations H11 or H13. The CNOMO specification X38CrMoV5.1 usedis similar to H11, but the CNOMO standardisation is specific.

PSA do not work with the H13 specification. All the testing PSAmadewith H13 until today, were not good in pressure die casting orin forging.

The demand asked by this standardisation is really difficultto reach. Thishas a cost of which PSA is aware, but problems caused by the useof lowerquality grades costs more.

CNOMO E01.17.221.N standard of the grade X38CrMoV5.1, is destinedto forging tools. CNOMO E01.17.222.N standardof the grade X36CrMoV5.1,is destined to pressure die casting tools. For these 2 grades, a chemical anal-ysis is determined, see Table 4.

At the reception of the tool steel delivery (mechanical characteristicsmaximum 220 HBW) typical microstructures are accepted, Figs. 1 and 2.Only A, B, C and D structures are accepted. E, F, G, H, I and J structuresare refused.

An important requirement to respect is the minimum level of ductility.


Table 4.

Grades C Mn Si S P Cr Mo V W Co Ni Cu

X38CrMoV5.1 Min 0,34 0,2 — — — 4,8 1,2 0,4 — — — —Max 0,42 0,5 1,2 0,01 0,025 5,5 1,5 0,6 0,2 0,2 0,2 0,2

X36CrMoV5.1 Min 0,32 0,2 — — — 4,75 1.2 0,4 — — — —Max 0,39 0,4 0,5 0,005 0,015 5.25 1,6 0,6 0,1 0,1 0,15 0,2

A B

C D

Figure 1. Accepted structure according to CNOMO – Magnification: X500.

The toughness obtained by impact toughness (KU) samples is determinedafter specific heat treatments as listed below.


E F G

H I J

Figure 2. Rejected structure according to CNOMO – Magnification: X5003-3) Heattreatment.

Quenching: temperature given by the supplier, time: 1 hour,with airor gas pressure cooling.

1 annealing: 550℃ during 1 hour

2 annealing: during 1 hour 30 minutes.

The hardness to reach is 48-50 HRC.

With these conditions the minimum impact toughness values are:

KU = 15 Joules for the X38CrMoV5.1


KU = 20 Joules for the X36CrMoV5.1.

This is important for the reasons given in the previous paragraphs.Sometimes it is very difficult to take a decision of acceptance or rejection

based on the microscope image of the microstructure. The KU test value isthe real judge for the quality level of the steel.

The quenching velocity is very important, since the nucleation of carbidesduring cooling is a well-known phenomena, Fig. 3. The carbides created atthe grain boundaries decrease the ductility of the steel.

Figure 3. TRC diagram of X38CrMoV5.1 (SECOSAR).

The faster the cooling, the better the material properties are. The invest-ment in a vacuum furnace with gas cooling, which presents a better thermalexchange than a cooling cell, is a good investment.


NITRIDING

In forging and in pressure die casting all tools are nitridedat PSA. In forg-ing three different types of nitriding are used, depending on the application.

Table 5.

Nitriding Type Length

single gas 0,2 + 0,05 mmdouble gas 0,3 + 0,1 mmtenifer salt 0,1 mm (10 µm of white layer)

In pressure die casting, the same nitriding methods are used.

Table 6.

Nitriding Type Lengthslow gas 0,1 mm

Nitriding is used by a lot of forgers to reduce the wear problem.However, nitriding in pressure die casting is not used a lot.It is the opin-

ion of PSA that nitriding can be dangerous, since it decreases the ductility ofthe material. A too heavy nitriding make the tool to go directly in the brittlemode at the surface, why, the lifetime of the tool can decrease very much.Therefore, it is important to control very carefully this thermochemical treat-ment. But, even if there is a risk, the nitriding treatment isperformed. In theexperience of PSA, if this treatment is not made the life timecan be dividedby 10. Nitriding increases the material resistance versus liquid aluminium,and it increases the yield point of the material. However, nitriding createscarbonitrides in the grain boundaries, which is the only problem and thereason why it is important to be sure of the treatment.


RESEARCH

FORGING

New development on steels is analysed at PSA. The objective is to increasethe tool steel hardness at high temperature, with a good level of ductility anda good thermal conductivity.

Work is made on repairing forging tools by welding. This application canbe very interesting, because it is possible to win 40 to 50 % ofthe cost ofthe tool per forging part (including the cost of the welding).

The objective is to repair the tool where it is broken, without changingthe entire tool. The experiment on this subject makes us think that:

The metal which presents the most interest is the superalloywith acobalt base. The application targeted is when the tool is affecteddangerously by temperature.

Making the welding on all the surface of the tool is not useful, it caneven be dangerous. As an example, on a connected rod matrix the firsttest was pushed on a complete welding. However, this did not workbecause of the bad conductivity of the superalloy.

It is important to weld only where necessary. As an example, the con-nected rod matrix was welded only where the tool was damaged.Thelifetime was increased and the problem of the bad thermal conductiv-ity of the superalloy was stopped, because the non-welded surface ofthe tool was still of steel.

The number of weld repairing is limited.

PRESSURE DIE CASTING

Like in forging, new developments are analysed at PSA. Any way, whichallows to increase the ductility are interesting, because it allows to increasethe hardness. Materials with better thermal conductivity are interesting too,because they decreases driving force of the thermal fatigueby reducing thetemperature gradient in the tool.


CONCLUSIONS

Improving the lifetime of a tool is a tricky business. When you thinkthat you solved a problem, you have got another who occurring. So theimprovement of the lifetime are always concentratednext from our standards.

The material approach is a good way. It is not the only one. Theprocessescan be optimized. In this perspective, the progress in lubricants, for example,have to be examined.

The material approach is nowadays mostly empiric. We are at the dawnof a really important progress in simulation. Thanks to that, we are going toknow all the mechanical and thermal conditions of how to use the tools. Weare beginning to follow a scientific approach. But it is just the beginning.

theuseofhot-worktoolsteelsatpsa peugeot citroËn … · theuseofhot-worktoolsteelsatpsa peugeot...

Documents